/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.locks;

import java.util.concurrent.TimeUnit;
import java.util.Collection;

/**
 * A reentrant mutual exclusion {@link Lock} with the same basic
 * behavior and semantics as the implicit monitor lock accessed using
 * {@code synchronized} methods and statements, but with extended
 * capabilities.
 *
 * <p>A {@code ReentrantLock} is <em>owned</em> by the thread last
 * successfully locking, but not yet unlocking it. A thread invoking
 * {@code lock} will return, successfully acquiring the lock, when
 * the lock is not owned by another thread. The method will return
 * immediately if the current thread already owns the lock. This can
 * be checked using methods {@link #isHeldByCurrentThread}, and {@link
 * #getHoldCount}.
 *
 * <p>The constructor for this class accepts an optional
 * <em>fairness</em> parameter.  When set {@code true}, under
 * contention, locks favor granting access to the longest-waiting
 * thread.  Otherwise this lock does not guarantee any particular
 * access order.  Programs using fair locks accessed by many threads
 * may display lower overall throughput (i.e., are slower; often much
 * slower) than those using the default setting, but have smaller
 * variances in times to obtain locks and guarantee lack of
 * starvation. Note however, that fairness of locks does not guarantee
 * fairness of thread scheduling. Thus, one of many threads using a
 * fair lock may obtain it multiple times in succession while other
 * active threads are not progressing and not currently holding the
 * lock.
 * Also note that the untimed {@link #tryLock()} method does not
 * honor the fairness setting. It will succeed if the lock
 * is available even if other threads are waiting.
 *
 * <p>It is recommended practice to <em>always</em> immediately
 * follow a call to {@code lock} with a {@code try} block, most
 * typically in a before/after construction such as:
 *
 * <pre> {@code
 * class X {
 *   private final ReentrantLock lock = new ReentrantLock();
 *   // ...
 *
 *   public void m() {
 *     lock.lock();  // block until condition holds
 *     try {
 *       // ... method body
 *     } finally {
 *       lock.unlock()
 *     }
 *   }
 * }}</pre>
 *
 * <p>In addition to implementing the {@link Lock} interface, this
 * class defines a number of {@code public} and {@code protected}
 * methods for inspecting the state of the lock.  Some of these
 * methods are only useful for instrumentation and monitoring.
 *
 * <p>Serialization of this class behaves in the same way as built-in
 * locks: a deserialized lock is in the unlocked state, regardless of
 * its state when serialized.
 *
 * <p>This lock supports a maximum of 2147483647 recursive locks by
 * the same thread. Attempts to exceed this limit result in
 * {@link Error} throws from locking methods.
 *
 * @author Doug Lea
 * @since 1.5
 */
public class ReentrantLock implements Lock, java.io.Serializable {

  private static final long serialVersionUID = 7373984872572414699L;
  /**
   * Synchronizer providing all implementation mechanics
   */
  private final Sync sync;

  /**
   * Base of synchronization control for this lock. Subclassed
   * into fair and nonfair versions below. Uses AQS state to
   * represent the number of holds on the lock.
   */
  abstract static class Sync extends AbstractQueuedSynchronizer {

    private static final long serialVersionUID = -5179523762034025860L;

    /**
     * Performs {@link Lock#lock}. The main reason for subclassing
     * is to allow fast path for nonfair version.
     */
    abstract void lock();

    /**
     * Performs non-fair tryLock.  tryAcquire is implemented in
     * subclasses, but both need nonfair try for trylock method.
     */
    final boolean nonfairTryAcquire(int acquires) {
      final Thread current = Thread.currentThread();
      int c = getState();
      if (c == 0) {
        /// 比公平锁的tryAcquire方法在第二个if判断中少了一个是否存在前继节点判断,  也就是说:
        // 公平锁在尝试获取锁时，即使“锁”没有被任何线程锁持有，它也会判断自己是不是CLH等待队列的表头；是的话，才获取锁。
        // 而非公平锁在尝试获取锁时，如果“锁”没有被任何线程持有，则不管它在CLH队列的何处，它都直接获取锁。
        if (compareAndSetState(0, acquires)) {
          setExclusiveOwnerThread(current);
          return true;
        }
      } else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0) // overflow
        {
          throw new Error("Maximum lock count exceeded");
        }
        setState(nextc);
        return true;
      }
      return false;
    }

    protected final boolean tryRelease(int releases) {
      int c = getState() - releases;
      if (Thread.currentThread() != getExclusiveOwnerThread()) {
        throw new IllegalMonitorStateException();
      }
      boolean free = false;
      if (c == 0) {
        free = true;
        setExclusiveOwnerThread(null);
      }
      setState(c);
      return free;
    }

    protected final boolean isHeldExclusively() {
      // While we must in general read state before owner,
      // we don't need to do so to check if current thread is owner
      return getExclusiveOwnerThread() == Thread.currentThread();
    }

    final ConditionObject newCondition() {
      return new ConditionObject();
    }

    // Methods relayed from outer class

    final Thread getOwner() {
      return getState() == 0 ? null : getExclusiveOwnerThread();
    }

    final int getHoldCount() {
      return isHeldExclusively() ? getState() : 0;
    }

    final boolean isLocked() {
      return getState() != 0;
    }

    /**
     * Reconstitutes the instance from a stream (that is, deserializes it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
      s.defaultReadObject();
      setState(0); // reset to unlocked state
    }
  }

  /**
   * Sync object for non-fair locks
   */
  static final class NonfairSync extends Sync {

    private static final long serialVersionUID = 7316153563782823691L;

    /**
     * Performs lock.  Try immediate barge, backing up to normal
     * acquire on failure.
     */
    /*/
     * 注意和公平方式的区别,
     * 公平锁 -- 公平锁的lock()函数，会直接调用acquire(1)。
     * 非公平锁 -- 非公平锁会先判断当前锁的状态是不是空闲，是的话，就不排队，而是直接获取锁。
     */
    final void lock() {
      if (compareAndSetState(0, 1)) {
        setExclusiveOwnerThread(Thread.currentThread());
      } else {
        acquire(1);
      }
    }

    protected final boolean tryAcquire(int acquires) {
      return nonfairTryAcquire(acquires);
    }
  }

  /**
   * Sync object for fair locks
   */
  static final class FairSync extends Sync {

    private static final long serialVersionUID = -3000897897090466540L;

    final void lock() {
      acquire(1);
    }

    /**
     * Fair version of tryAcquire.  Don't grant access unless
     * recursive call or no waiters or is first.
     */
    protected final boolean tryAcquire(int acquires) {
      final Thread current = Thread.currentThread();
      int c = getState();
      /// state表示加锁的次数
      if (c == 0) {
        /*/
         * 首先判断是否有前继结点，如果没有则当前队列中还没有其他线程；
         * 设置状态为acquires，即lock方法中写死的1,
         * 这里为什么不直接setState？因为可能同时有多个线程同时在执行到此处，所以用CAS来执行；
         * 设置当前线程独占锁。
         */
        if (!hasQueuedPredecessors() &&
            compareAndSetState(0, acquires)) {
          setExclusiveOwnerThread(current);
          return true;
        }
      } else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0) {
          throw new Error("Maximum lock count exceeded");
        }
        /// 为什么不用compareAndSetState？因为独占锁的线程已经是当前线程，不需要通过CAS来设置。
        setState(nextc);
        return true;
      }
      return false;
    }
  }

  /**
   * Creates an instance of {@code ReentrantLock}.
   * This is equivalent to using {@code ReentrantLock(false)}.
   */
  public ReentrantLock() {
    sync = new NonfairSync();
  }

  /**
   * Creates an instance of {@code ReentrantLock} with the
   * given fairness policy.
   *
   * @param fair {@code true} if this lock should use a fair ordering policy
   */
  public ReentrantLock(boolean fair) {
    sync = fair ? new FairSync() : new NonfairSync();
  }

  /**
   * Acquires the lock.
   *
   * <p>Acquires the lock if it is not held by another thread and returns
   * immediately, setting the lock hold count to one.
   *
   * <p>If the current thread already holds the lock then the hold
   * count is incremented by one and the method returns immediately.
   *
   * <p>If the lock is held by another thread then the
   * current thread becomes disabled for thread scheduling
   * purposes and lies dormant until the lock has been acquired,
   * at which time the lock hold count is set to one.
   */
  public void lock() {
    sync.lock();
  }

  /**
   * Acquires the lock unless the current thread is
   * {@linkplain Thread#interrupt interrupted}.
   *
   * <p>Acquires the lock if it is not held by another thread and returns
   * immediately, setting the lock hold count to one.
   *
   * <p>If the current thread already holds this lock then the hold count
   * is incremented by one and the method returns immediately.
   *
   * <p>If the lock is held by another thread then the
   * current thread becomes disabled for thread scheduling
   * purposes and lies dormant until one of two things happens:
   *
   * <ul>
   *
   * <li>The lock is acquired by the current thread; or
   *
   * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
   * current thread.
   *
   * </ul>
   *
   * <p>If the lock is acquired by the current thread then the lock hold
   * count is set to one.
   *
   * <p>If the current thread:
   *
   * <ul>
   *
   * <li>has its interrupted status set on entry to this method; or
   *
   * <li>is {@linkplain Thread#interrupt interrupted} while acquiring
   * the lock,
   *
   * </ul>
   *
   * then {@link InterruptedException} is thrown and the current thread's
   * interrupted status is cleared.
   *
   * <p>In this implementation, as this method is an explicit
   * interruption point, preference is given to responding to the
   * interrupt over normal or reentrant acquisition of the lock.
   *
   * @throws InterruptedException if the current thread is interrupted
   */
  public void lockInterruptibly() throws InterruptedException {
    sync.acquireInterruptibly(1);
  }

  /**
   * Acquires the lock only if it is not held by another thread at the time
   * of invocation.
   *
   * <p>Acquires the lock if it is not held by another thread and
   * returns immediately with the value {@code true}, setting the
   * lock hold count to one. Even when this lock has been set to use a
   * fair ordering policy, a call to {@code tryLock()} <em>will</em>
   * immediately acquire the lock if it is available, whether or not
   * other threads are currently waiting for the lock.
   * This &quot;barging&quot; behavior can be useful in certain
   * circumstances, even though it breaks fairness. If you want to honor
   * the fairness setting for this lock, then use
   * {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
   * which is almost equivalent (it also detects interruption).
   *
   * <p>If the current thread already holds this lock then the hold
   * count is incremented by one and the method returns {@code true}.
   *
   * <p>If the lock is held by another thread then this method will return
   * immediately with the value {@code false}.
   *
   * @return {@code true} if the lock was free and was acquired by the current thread, or the lock
   * was already held by the current thread; and {@code false} otherwise
   */
  public boolean tryLock() {
    return sync.nonfairTryAcquire(1);
  }

  /**
   * Acquires the lock if it is not held by another thread within the given
   * waiting time and the current thread has not been
   * {@linkplain Thread#interrupt interrupted}.
   *
   * <p>Acquires the lock if it is not held by another thread and returns
   * immediately with the value {@code true}, setting the lock hold count
   * to one. If this lock has been set to use a fair ordering policy then
   * an available lock <em>will not</em> be acquired if any other threads
   * are waiting for the lock. This is in contrast to the {@link #tryLock()}
   * method. If you want a timed {@code tryLock} that does permit barging on
   * a fair lock then combine the timed and un-timed forms together:
   *
   * <pre> {@code
   * if (lock.tryLock() ||
   *     lock.tryLock(timeout, unit)) {
   *   ...
   * }}</pre>
   *
   * <p>If the current thread
   * already holds this lock then the hold count is incremented by one and
   * the method returns {@code true}.
   *
   * <p>If the lock is held by another thread then the
   * current thread becomes disabled for thread scheduling
   * purposes and lies dormant until one of three things happens:
   *
   * <ul>
   *
   * <li>The lock is acquired by the current thread; or
   *
   * <li>Some other thread {@linkplain Thread#interrupt interrupts}
   * the current thread; or
   *
   * <li>The specified waiting time elapses
   *
   * </ul>
   *
   * <p>If the lock is acquired then the value {@code true} is returned and
   * the lock hold count is set to one.
   *
   * <p>If the current thread:
   *
   * <ul>
   *
   * <li>has its interrupted status set on entry to this method; or
   *
   * <li>is {@linkplain Thread#interrupt interrupted} while
   * acquiring the lock,
   *
   * </ul>
   * then {@link InterruptedException} is thrown and the current thread's
   * interrupted status is cleared.
   *
   * <p>If the specified waiting time elapses then the value {@code false}
   * is returned.  If the time is less than or equal to zero, the method
   * will not wait at all.
   *
   * <p>In this implementation, as this method is an explicit
   * interruption point, preference is given to responding to the
   * interrupt over normal or reentrant acquisition of the lock, and
   * over reporting the elapse of the waiting time.
   *
   * @param timeout the time to wait for the lock
   * @param unit    the time unit of the timeout argument
   * @return {@code true} if the lock was free and was acquired by the current thread, or the lock
   * was already held by the current thread; and {@code false} if the waiting time elapsed before
   * the lock could be acquired
   * @throws InterruptedException if the current thread is interrupted
   * @throws NullPointerException if the time unit is null
   */
  public boolean tryLock(long timeout, TimeUnit unit)
      throws InterruptedException {
    return sync.tryAcquireNanos(1, unit.toNanos(timeout));
  }

  /**
   * Attempts to release this lock.
   *
   * <p>If the current thread is the holder of this lock then the hold
   * count is decremented.  If the hold count is now zero then the lock
   * is released.  If the current thread is not the holder of this
   * lock then {@link IllegalMonitorStateException} is thrown.
   *
   * @throws IllegalMonitorStateException if the current thread does not hold this lock
   */
  public void unlock() {
    sync.release(1);
  }

  /**
   * Returns a {@link Condition} instance for use with this
   * {@link Lock} instance.
   *
   * <p>The returned {@link Condition} instance supports the same
   * usages as do the {@link Object} monitor methods ({@link
   * Object#wait() wait}, {@link Object#notify notify}, and {@link
   * Object#notifyAll notifyAll}) when used with the built-in
   * monitor lock.
   *
   * <ul>
   *
   * <li>If this lock is not held when any of the {@link Condition}
   * {@linkplain Condition#await() waiting} or {@linkplain
   * Condition#signal signalling} methods are called, then an {@link
   * IllegalMonitorStateException} is thrown.
   *
   * <li>When the condition {@linkplain Condition#await() waiting}
   * methods are called the lock is released and, before they
   * return, the lock is reacquired and the lock hold count restored
   * to what it was when the method was called.
   *
   * <li>If a thread is {@linkplain Thread#interrupt interrupted}
   * while waiting then the wait will terminate, an {@link
   * InterruptedException} will be thrown, and the thread's
   * interrupted status will be cleared.
   *
   * <li> Waiting threads are signalled in FIFO order.
   *
   * <li>The ordering of lock reacquisition for threads returning
   * from waiting methods is the same as for threads initially
   * acquiring the lock, which is in the default case not specified,
   * but for <em>fair</em> locks favors those threads that have been
   * waiting the longest.
   *
   * </ul>
   *
   * @return the Condition object
   */
  public Condition newCondition() {
    return sync.newCondition();
  }

  /**
   * Queries the number of holds on this lock by the current thread.
   *
   * <p>A thread has a hold on a lock for each lock action that is not
   * matched by an unlock action.
   *
   * <p>The hold count information is typically only used for testing and
   * debugging purposes. For example, if a certain section of code should
   * not be entered with the lock already held then we can assert that
   * fact:
   *
   * <pre> {@code
   * class X {
   *   ReentrantLock lock = new ReentrantLock();
   *   // ...
   *   public void m() {
   *     assert lock.getHoldCount() == 0;
   *     lock.lock();
   *     try {
   *       // ... method body
   *     } finally {
   *       lock.unlock();
   *     }
   *   }
   * }}</pre>
   *
   * @return the number of holds on this lock by the current thread, or zero if this lock is not
   * held by the current thread
   */
  public int getHoldCount() {
    return sync.getHoldCount();
  }

  /**
   * Queries if this lock is held by the current thread.
   *
   * <p>Analogous to the {@link Thread#holdsLock(Object)} method for
   * built-in monitor locks, this method is typically used for
   * debugging and testing. For example, a method that should only be
   * called while a lock is held can assert that this is the case:
   *
   * <pre> {@code
   * class X {
   *   ReentrantLock lock = new ReentrantLock();
   *   // ...
   *
   *   public void m() {
   *       assert lock.isHeldByCurrentThread();
   *       // ... method body
   *   }
   * }}</pre>
   *
   * <p>It can also be used to ensure that a reentrant lock is used
   * in a non-reentrant manner, for example:
   *
   * <pre> {@code
   * class X {
   *   ReentrantLock lock = new ReentrantLock();
   *   // ...
   *
   *   public void m() {
   *       assert !lock.isHeldByCurrentThread();
   *       lock.lock();
   *       try {
   *           // ... method body
   *       } finally {
   *           lock.unlock();
   *       }
   *   }
   * }}</pre>
   *
   * @return {@code true} if current thread holds this lock and {@code false} otherwise
   */
  public boolean isHeldByCurrentThread() {
    return sync.isHeldExclusively();
  }

  /**
   * Queries if this lock is held by any thread. This method is
   * designed for use in monitoring of the system state,
   * not for synchronization control.
   *
   * @return {@code true} if any thread holds this lock and {@code false} otherwise
   */
  public boolean isLocked() {
    return sync.isLocked();
  }

  /**
   * Returns {@code true} if this lock has fairness set true.
   *
   * @return {@code true} if this lock has fairness set true
   */
  public final boolean isFair() {
    return sync instanceof FairSync;
  }

  /**
   * Returns the thread that currently owns this lock, or
   * {@code null} if not owned. When this method is called by a
   * thread that is not the owner, the return value reflects a
   * best-effort approximation of current lock status. For example,
   * the owner may be momentarily {@code null} even if there are
   * threads trying to acquire the lock but have not yet done so.
   * This method is designed to facilitate construction of
   * subclasses that provide more extensive lock monitoring
   * facilities.
   *
   * @return the owner, or {@code null} if not owned
   */
  protected Thread getOwner() {
    return sync.getOwner();
  }

  /**
   * Queries whether any threads are waiting to acquire this lock. Note that
   * because cancellations may occur at any time, a {@code true}
   * return does not guarantee that any other thread will ever
   * acquire this lock.  This method is designed primarily for use in
   * monitoring of the system state.
   *
   * @return {@code true} if there may be other threads waiting to acquire the lock
   */
  public final boolean hasQueuedThreads() {
    return sync.hasQueuedThreads();
  }

  /**
   * Queries whether the given thread is waiting to acquire this
   * lock. Note that because cancellations may occur at any time, a
   * {@code true} return does not guarantee that this thread
   * will ever acquire this lock.  This method is designed primarily for use
   * in monitoring of the system state.
   *
   * @param thread the thread
   * @return {@code true} if the given thread is queued waiting for this lock
   * @throws NullPointerException if the thread is null
   */
  public final boolean hasQueuedThread(Thread thread) {
    return sync.isQueued(thread);
  }

  /**
   * Returns an estimate of the number of threads waiting to
   * acquire this lock.  The value is only an estimate because the number of
   * threads may change dynamically while this method traverses
   * internal data structures.  This method is designed for use in
   * monitoring of the system state, not for synchronization
   * control.
   *
   * @return the estimated number of threads waiting for this lock
   */
  public final int getQueueLength() {
    return sync.getQueueLength();
  }

  /**
   * Returns a collection containing threads that may be waiting to
   * acquire this lock.  Because the actual set of threads may change
   * dynamically while constructing this result, the returned
   * collection is only a best-effort estimate.  The elements of the
   * returned collection are in no particular order.  This method is
   * designed to facilitate construction of subclasses that provide
   * more extensive monitoring facilities.
   *
   * @return the collection of threads
   */
  protected Collection<Thread> getQueuedThreads() {
    return sync.getQueuedThreads();
  }

  /**
   * Queries whether any threads are waiting on the given condition
   * associated with this lock. Note that because timeouts and
   * interrupts may occur at any time, a {@code true} return does
   * not guarantee that a future {@code signal} will awaken any
   * threads.  This method is designed primarily for use in
   * monitoring of the system state.
   *
   * @param condition the condition
   * @return {@code true} if there are any waiting threads
   * @throws IllegalMonitorStateException if this lock is not held
   * @throws IllegalArgumentException     if the given condition is not associated with this lock
   * @throws NullPointerException         if the condition is null
   */
  public boolean hasWaiters(Condition condition) {
    if (condition == null) {
      throw new NullPointerException();
    }
    if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) {
      throw new IllegalArgumentException("not owner");
    }
    return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject) condition);
  }

  /**
   * Returns an estimate of the number of threads waiting on the
   * given condition associated with this lock. Note that because
   * timeouts and interrupts may occur at any time, the estimate
   * serves only as an upper bound on the actual number of waiters.
   * This method is designed for use in monitoring of the system
   * state, not for synchronization control.
   *
   * @param condition the condition
   * @return the estimated number of waiting threads
   * @throws IllegalMonitorStateException if this lock is not held
   * @throws IllegalArgumentException     if the given condition is not associated with this lock
   * @throws NullPointerException         if the condition is null
   */
  public int getWaitQueueLength(Condition condition) {
    if (condition == null) {
      throw new NullPointerException();
    }
    if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) {
      throw new IllegalArgumentException("not owner");
    }
    return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject) condition);
  }

  /**
   * Returns a collection containing those threads that may be
   * waiting on the given condition associated with this lock.
   * Because the actual set of threads may change dynamically while
   * constructing this result, the returned collection is only a
   * best-effort estimate. The elements of the returned collection
   * are in no particular order.  This method is designed to
   * facilitate construction of subclasses that provide more
   * extensive condition monitoring facilities.
   *
   * @param condition the condition
   * @return the collection of threads
   * @throws IllegalMonitorStateException if this lock is not held
   * @throws IllegalArgumentException     if the given condition is not associated with this lock
   * @throws NullPointerException         if the condition is null
   */
  protected Collection<Thread> getWaitingThreads(Condition condition) {
    if (condition == null) {
      throw new NullPointerException();
    }
    if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) {
      throw new IllegalArgumentException("not owner");
    }
    return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject) condition);
  }

  /**
   * Returns a string identifying this lock, as well as its lock state.
   * The state, in brackets, includes either the String {@code "Unlocked"}
   * or the String {@code "Locked by"} followed by the
   * {@linkplain Thread#getName name} of the owning thread.
   *
   * @return a string identifying this lock, as well as its lock state
   */
  public String toString() {
    Thread o = sync.getOwner();
    return super.toString() + ((o == null) ?
        "[Unlocked]" :
        "[Locked by thread " + o.getName() + "]");
  }
}
